WHOLE-CELL AND SINGLE-CHANNEL ANALYSIS OF THE KINETICS OF GLYCINE-SENSITIVE N-METHYL-D-ASPARTATE RECEPTOR DESENSITIZATION

1 The kinetics of glycine-sensitive, N-methyl-D-aspartate (NMDA) recep tor desensitization were investigated in cultured neurones with the pa tch clamp technique. 2 The degree of fast NMDA-receptor desensitizatio n was inversely related to glycine concentration. Thus, increasing con centrations of glycine from 30 nm to 2.5 mum potentiated desensitized NMDA responses (873% +/- 101%) to a greater degree than peak responses (260% +/-27%). 3 The desensitization was due to a decrease in the aff inity of glycine for the strychnine-insensitive, glycine modulatory si te (glycine(B) site) following activation of the NMDA-receptor complex . Thus, the A50 for glycine in potentiating peak responses (77 nm, 95% confidence limited 58-104 nm) was five fold lower than that for plate au responses (399 nm, 340-468 nm). 4 The rate of desensitization was r elated to glycine concentration such that a reciprocal plot of desensi tization rate (1/tau S-1) against glycine concentration had a slope of 9.510(6) M-1 S-1. 5 Recovery from desensitization following step inc reases in glycine or L-alanine concentration in the continuous presenc e of NMDA (200 mum) reflected the association kinetics of the glycine, agonist used. 6 The rate and degree of NMDA receptor desensitization was independent of holding potential. 7 NMDA receptor desensitization was also evident at the single channel level. 8 The glycine(B) antagon ist 7-chlorokynurenic acid (7-Chl-Kyn 3 and 10 mum) concentration-depe ndently induced an identical form of desensitization in the presence o f 1 mum glycine. 9 In contrast, the competitive NMDA antagonist (+/-)- amino-phosphonovaleric acid (APV 30 to 300 mum) concentration-dependen tly antagonized and slowed the onset kinetics of NMDA responses.